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1. Field of the Invention
The present invention relates generally to high speed digital data communications for use in downhole telemetry. More specifically, the invention relates to a communication scheme for transferring data from downhole sensors to surface computers. More specifically still, the invention relates to multiple modems transmitting data to a single central modem where each of the multiple modems communicates to the central modem in an allocated band of frequencies of an overall bandwidth of a communication channel.
2. Background of the Invention
Information is the key to being profitable in the oil and gas industry. The more information one has regarding location and migration patterns of hydrocarbons within a hydrocarbon reservoir, the more likely it is that that reservoir can be tapped at its optimal location and utilized to its full potential.
One mechanism for obtaining information about hydrocarbon reservoirs is the use of seismic imaging. Classic seismic imaging involves stringing hundreds of listening devices, or geophones, over the surface of the Earth near a location where a characteristic picture of the underground formations is desired. Geophones measure both compressional and shear waves directly and they include particle velocity detectors. Geophones typically provide three-component velocity measurement. Geophones can be used to determine the direction of arrival of incident elastic waves. Once these geophones are strategically placed on the surface of the Earth, a seismic disturbance is created which creates traveling waves through the Earth""s crust. As these traveling waves encounter boundaries of strata having varying densities, portions of the traveling wave reflect back to the surface. These varying density stratas may include changes in strata components as well as varying densities encountered at boundaries of hydrocarbon reservoirs. By measuring the propagation time, amplitude and direction of reflected waves as they reach the surface, a three-dimensional representation of the formations lying below the surface of the Earth can be constructed.
After a particular hydrocarbon formation is found, the need for information is not alleviated. Once a hydrocarbon reservoir is tapped, the goal becomes removing as much of the hydrocarbons from the reservoir as possible. Here again, the more information one has about the locations of hydrocarbons within the reservoir over the course of time, the more likely the hydrocarbons contained in the reservoir can be fully extracted at the lowest possible cost. Having multiple three-dimensional seismic representations of conditions below the surface over time is typically referred to as four-dimensional (4D) seismic imaging. In early implementations, four-dimensional seismic was created by performing multiple three-dimensional seismic images of the strata or hydrocarbon reservoir in question. However, obtaining four-dimensional seismic representations of underground hydrocarbon reservoirs in this manner has its problems. For instance, the time period for taking readings to determine migration patterns of the hydrocarbons may be as long as years. That is, a single three-dimensional seismic reading may be taken once a year over the course of several years to obtain the four-dimensional seismic image. Each time this three-dimensional seismic image is taken, miles of cables containing geophones must be laid on the surface of the Earth. It is almost impossible to lay these cables in exactly the same location between each three-dimensional imaging session and further, even if the cables are placed relatively close to their locations from previous measurements, the geophones within the cable themselves may not be physically located the same as in previous three-dimensional images.
One way to combat these problems is to place the geophones vertically instead of horizontally. Rather than stretching cables across the surface of the Earth to place the geophones in a relatively horizontal position, the geophones themselves are semi-permanently lowered into well bores such that they are oriented vertically with respect to the surface of the Earth. The well bore into which the geophones may be lowered could be, for example, an existing oil or gas well or may be a well bore dedicated to sensor installation. While permanent placement of the geophones. in a well bore may solve the placement problem for four-dimensional seismic imaging, new problems arise.
Given that seismic imaging fundamentally is measuring the arrival time of reflected waves at one location relative to arrival of reflected waves at another location, knowing the arrival time of all reflected waves relative to each other is critical to the computationally heavy burden of reconstructing an image of the below ground structures over time. To accomplish this task, large quantities of information must be recorded, substantially simultaneously, to correlate the arrival time of the various reflected waves. For traditional 3D seismic operations, whose sensors are simply laidon the surface of the Earth, having sufficient physical space necessary to communicate with each geophone is not a concern. For example, each geophone may have a single twisted pair cable, dedicated just to that geophone, coupled to a computer such that the computer can read each geophone substantially simultaneously. However, when permanently installing geophones in a vertical orientation in a well bore, physical space is not in abundance and therefore having a dedicated twisted pair cable for each geophone may not be feasible. Indeed, having a cable with a dedicated twisted pair for each geophone in a vertically oriented system, which may have assmany as two hundred geophones, may require more cable cross-sectional area than the borehole itself.
For these reasons it would be desirable to have a communication technique that could support communication to the number of geophones necessary for 4D seismic imaging without unnecessarily large communication cables.
The problems noted above are solved in large part by a communication system that uses a modified orthogonal frequency division multiplexing scheme to divide the available bandwidth of each twisted pair cable into discrete frequency bands or sub-channels. Each downhole sensor has associated with it a modem capable of communicating over any of the discrete frequency sub-channels; however, each downhole modem is allocated only a few of the discrete frequency sub-channels such that each downhole modem associated with a seismic sensor can communicate substantially simultaneously to a surface modem. Data is sent to the surface in super-frames created by a group downhole modems each transmitting data on its allocated frequencies in a shared frame period. The information transmitted by each downhole modem in its allocated frequencies within the frame period arrives at the surface modem and combines to form a super-frame, and the surface modem discriminates and decodes the information. The super-frame created by the downhole modems is a modification of the super-frame created by the Assymetrical Digital Subscriber Line (ADSL) standard for data framing, which applies only to point to point communications.
In one embodiment, a single twisted pair cable communicates to as many as two hundred downhole modems associated with seismic sensors. Using these discrete frequency sub-channels for data communication from the downhole modems to the surface modem, a combined data rate of about 6 to 8 megabits per second can be communicated up hole. By compressing data downhole and having all the downhole modems communicate over a single twistedpair cable, the desired data rates for 4D seismic, which exceed 30 megabits per second, can be achieved.
In a second embodiment, no data compression is used. Instead, five twisted pair cables are grouped into a single overall cable. Each single twisted pair in the group is preferably associated with approximately forty downhole sensors and, since each single twisted pair can carry approximately 6-8 megabits per second, the overall data transmission rate for the five twisted pair cables exceeds the 4D seismic data transmission rate of about 30 megabits per second.